Beaufort scale: wind force at sea, from 0 to 12

The Beaufort scale is the common rule all sailors use to talk about the wind. When a forecast announces “westerly force 4 to 5”, every skipper knows what to expect: a nice breeze, whitecaps on the water and a first reef to plan for. This page brings together everything you need to know about wind force at sea: the full Beaufort scale table from 0 to 12, the conversion from knots to km/h, the corresponding wave height, the story of Admiral Beaufort and the way we, skippers in Marseille, use this scale to decide whether a trip goes ahead.

Drag the simulator slider to watch the sea build, from the mirror of force 0 to the white breakers of force 10. Click a row of the table to display it in the simulator.

The beauty of the Beaufort scale is that it can be read without instruments. Long before anemometers, sailors estimated wind force by looking at the surface of the water. These benchmarks remain valuable today: they let you check at a glance whether the forecast matches reality, before leaving port.

Ripples and wavelets: forces 1 to 3

In very light wind, the sea is covered with small scale-like ripples, without foam. At force 2, the wavelets have smooth crests that glint in the sun. At force 3, a few crests begin to break: you see the first whitecaps, those small scattered patches of white foam.

Whitecaps: forces 4 and 5

Whitecaps are the best indicator of wind force at sea. Frequent at force 4, they cover the whole surface at force 5, where the waves lengthen and the first spray flies. If you see whitecaps all over the harbour, count on at least 17 knots of wind.

Foam in streaks: forces 6 to 8

At force 6, large waves form and the foam crests become wide and white. At force 7, foam torn from the crests stretches into streaks along the wind. At force 8, these streaks form well-marked bands: this is a gale, and the sea becomes dangerous for a pleasure boat.

A sea turning white: forces 9 to 12

Beyond that, spray reduces visibility, waves break in rollers and the sea surface turns white. These extreme forces are rare in the Mediterranean near the coast, but the Gulf of Lion sees mistral gales of force 8 to 9 in winter.

Note that the wave heights in the table are probable values in open sea, when the wind has been blowing for a long time over a great distance. Near the coast, the sea also depends on the wind direction: an offshore wind leaves the water flat along the shore, an onshore wind quickly builds it up.

Météo-France marine forecasts express the wind in Beaufort force, with its direction (where it comes from) and how it will change. A forecast of “north-westerly 4 to 5, gusts 6, slight to moderate sea” reads as follows: north-westerly wind between 11 and 21 knots, gusts of 27 knots, waves of 0.5 to 2.5 metres.

Near-gale, gale and storm warnings

When the wind is expected to reach force 7 in a coastal area, Météo-France issues a special weather bulletin, the coastal BMS. It is called a near-gale warning at force 7, a gale warning at force 8 and a storm warning from force 10. These warnings are relayed by CROSS on VHF after a call on channel 16. For a pleasure boat, a BMS in force is reason enough not to go out.

Wind force and sea state: the Douglas scale

The sea state is described by another scale, the Douglas scale, based on wave height:

Swell, generated by a distant wind, adds to the sea raised by the local wind. An easterly swell can make the sea moderate in Marseille while the wind is light: that is why a good forecast always gives both the wind and the sea state.

On a sailboat, wind force translates directly into sail trim. Up to force 4, you sail with full canvas and the boat accelerates with every gust. At force 5, the heel becomes strong: you take a first reef in the mainsail and reduce the genoa to keep the boat comfortable and manoeuvrable. At force 6, two reefs and a well-furled genoa are the rule, and every manoeuvre is prepared.

Beyond that, sail is reduced further down to the storm jib, then bare poles. The simulator on this page shows this progression: watch the mainsail shrink as the force increases. Above all, remember the sailors’ rule: reef as soon as you wonder whether you should.

Wind force is not everything. The duration of the blow, the direction relative to the coast, the sea state, the size of the boat and the crew’s experience matter just as much. A force 5 mistral in the shelter of the Côte Bleue has nothing in common with a force 5 easterly off the calanques.